The present invention relates to plastics material integral mesh structures having mesh openings defined by a generally rectangular grid of substantially parallel, orientated strands and junctions therebetween, and also to an intermediate structure which is produced when making the final structure and which has in itself commercial uses.
A problem in all integral mesh structures relates to the junctions. The junctions should be sufficiently strong without containing too much plastics material.
U.K. patent specification No. 982 036 describes the production of rectangular grid mesh structures by stretching a substantially monoplanar plastics material sheet comprising a pattern of holes whose centres lie on a rectangular grid; the sheet is stretched biaxially in two directions parallel to the axes of the grid. The junctions so produced are not stretched and are thick and heavy.
U.K. patent specification No. 1 310 474 describes structures whose junctions are strong enough for the light duty applications with which the Patent Specification is concerned but are not strong enough for heavy duty applications. FIG. 5 of the Patent Specification shows two pairs of strands running into the junction, and the junction is formed of crotch filaments, a centre filament and thin webs. The webs may be about half the thickness of the strands. It is found that on rupture, the structure very often breaks at the junction, a split starting in a web, which acts as a tear starter, and running along the strands. The alternative structure illustrated in FIG. 4 is very similar except that the centre filament is absent, and its behaviour is similar.
U.K. patent specification No. 1 544 608 describes structures which are thin and flat, the junctions being no thicker than the strands. It is believed that a flat junction is not the strongest junction as, at the very centre of the junction, there is an area of random molecular arrangement which has less resistance to rupture than the molecularly orientated areas which surround it; the surrounding areas have the same thickness as, and are therefore stronger than, the centre area.
Definitions
The term "rectangular" includes square.
The term "orientated" means molecularly orientated.
The terms "rows" and "columns" are used for convenience to denote the axes of the rectangular grid. Unless specified otherwise, it does not matter if the rows extend in the machine direction (MD) or in the transverse direction (TD).
The terms "thick", "thin", "thickness", "deep", "depth", and "shallow" refer to the dimension normal to the plane of the starting material or mesh structure and the terms "wide", "narrow", and "width" refer to the appropriate dimension in the plane of the starting material or mesh structure.
The thickness of the starting material or mesh structure is the distance between the extreme faces of the starting material or mesh structure.
The thickness or depth of a strand is the thickness of the strand cross-section, but ignoring raised edges. Particularly if the original holes or depressions have no radiussing where they issue at the faces of the starting material, the strands will have a "pin-cushion" cross-section, with raised edges and lower centres; the thickness or depth will be as measured inwardly of the raised edges.
The notional junction zones of the starting material are notional zones defined by the intersection of the notional parallel-sided zone which lies between and is tangential to two columns of holes or depressions and the notional parallel-sided zone which lies between and is tangential to two rows of holes or depressions.
The term "tangential" includes touching, but not intersecting, a straight sided hole, depression or opening or touching a corner of a hole, depression or opening.
The depressions are not necessarily formed by the application of pressure.
Stretch ratios are either given overall or "on the strands". If they are given on the strands, they are measured by measuring the distance moved by the respective ends of the openings on either side of the strand. For the second stretch, the ratios are measured by comparing the stretched lengths with the original starting material and not with the material as after the first stretch. The ratios are as measured after relaxation.